The present invention relates to a hydraulic power booster for the actuation of a master cylinder in a hydraulic brake system for motor vehicles having a pressure fluid source. The invention relates to a hydraulic booster including a control device for the control of the hydraulic pressure level in a booster pressure chamber, an actuating element for the actuation of the control device wherein the actuating element is acted upon by an input force, a device for converting the hydraulic pressure into a translatory force wherien the converting device is acted upon by a pressure fluid and a force transmission element for transmitting the translatory force onto other force transmission elements connected to the master cylinder.
Hydraulic power boosters of this type are being used in increasingly large numbers in hydraulic actuating systems for motor vehicle brakes. They are used in particular as power boosters in anti-lock control systems and traction slip control systems for motor vehicles.
In automobile design, the hydraulic brake power booster competes with the conventional vacuum brake power booster.
The vacuum brake power booster still has certain advantages over the hydraulic brake power booster, in particular in view of its operating convenience. On the other hand, the hydraulic brake power booster has become indispensable in many fields of application.
There is, therefore, a need to improve the operating convenience of the hydraulic brake power booster so that it will at least meet and preferably exceed the operating convenience of the vacuum brake power booster.
When comparing the differences in convenience more closely one will note that in conventional hydrualic brake power boosters, as compared with vacuum brake power boosters, a dull pedal feel is often encountered in the lower working range. This is due to the fact that higher response forces must be produced in the hydraulic brake power booster as compared with the vacuum brake power booster during the initial phase of the brake actuation.
During the development of the vacuum brake power booster, the same problem was also encountered in the vacuum brake power booster. The problem was solved by a structure known as the so-called two-stage effect, or two-stage function.
This two-stage function of the vacuum brake power booster can also be put into practice according to the following design:
By applying only a very low actuating force onto the brake pedal, first a valve is opened in the vacuum brake power booster in order to initiate the vacuum booster function of the vacuum brake power booster. The servo force thus generated actuates the tandem master cylinder. An initial pressure of approx. 5 bar is built up in the hydrualic brake circuits.
This initial pressure build-up is thus achieved without any significant physical exertion, that is, without a significant pedal force and the pedal actuation is correspondingly relatively easy.
The driver will only note the actual reaction force at the brake pedal upon completion of the two-stage function.
In a known design, a reaction disc made of rubber is clamped between the valve piston of the vacuum brake power booster and the operating rod of the tandem master cylinder piston.
Upon actuation of the brake pedal, the pedal force is transmitted from the brake pedal by the pedal piston rod and the actuating piston of the vacuum brake power booster onto the rubber reaction disc. The force deforms the rubber reaction disc. The degree of deformation is predetermined and occurs in an axial direction. In automotive brake engineering, this predetermined degree of deformation is called "Z-measure" and is one of the design parameters for brake power boosters.
After the "Z-measure" has been overcome, the actual vacuum brake power booster function starts. The booster increases the pressure in the tandem master cylinder and thus in the hydraulic actuating system in dependence upon the input force, that is, the pedal force.
In the known device, the degree of two-stage effect can be set and depends on the Shore hardness of the rubber of the reaction disc and on the "Z-measure".
The German printed and published patent application P 3542418.4 (corresponding to U.S. Pat. No. 4,750,406 issued June 14, 1988) discloses a hydraulic booster for the actuation of master cylinders in motor vehicle brake systems. This booster has been designed such that the booster piston is provided with a bore at its end opposite the pedal wherein an auxiliary piston is longitudinally displaceably and sealingly arranged, which auxiliary piston is effectively connected with the piston of the master cylinder and confines, together with the booster piston, a chamber which communicates with the booster chamber through a channel provided in the booster piston.